1. Field of the Invention
The present invention relates to a magnetic encoder constituting a rotation detection apparatus for detecting the rotation number of the wheel for automobiles and more particularly relates to a magnetic encoder provided to detect the rotation number of the wheel in order to be applied to an antilock brake system or a traction control system of automobiles.
2. Prior Art
A magnetic encoder comprised of a metal reinforcing ring fitted and fixed to a rotary side member of the wheel for automobile and a circular multipolar magnet fixed to the metal reinforcing ring provided so as to face the magnetic sensor at the vehicle body side is applied. The circular multipolar magnet is formed into a magnetic encoder such that a magnetic powder such as ferrite is mixed and contained in rubber or a resin material, the mixture is molded separately from the metal reinforcing ring, the molded product is fixed and integrated with the metal reinforcing ring with an adhesive, and the product is magnetized by providing a plurality of N poles and S poles alternately.
Thus formed magnetic encoder is apt to be exposed in a hard environment where the foreign materials such as water, mud, dust and the like are scattered when it is used for detecting the rotation of the automobile wheels, so that a high durability is required. A thermal shock test is executed by alternately repeating high temperature conditions and low temperature conditions in order to examine the durability as a product. The circular multipolar magnet may be cracked or distortion or gap may be caused between the metal reinforcing ring and the circular multipolar magnet by the difference of the linear expansion rate of the metal reinforcing ring and the circular multipolar magnet.
In order to solve such problems, it can think up to obtain a thickness of the adhesive layer to fix the metal reinforcing ring and the circular multipolar magnet in order to absorb the thermal expansion difference therebetween.
The following patent document 1 discloses a magnetic encoder in which a reinforcing ring (metal reinforcing ring) and a magnetic ring are fixed with an adhesive and thereafter they are magnetized by providing S poles and N poles alternately along the circumference. The following patent document 2 discloses that a groove is formed on the adhesion face of an encoder (circular multipolar magnet) to a fitting member (metal reinforcing ring) fixed to a rotary body, said encoder being formed of a magnet which is circular and is magnetized in multipolar along the circumferential direction.
[Patent document 1] JP-2004-138597-A
[Patent document 2] JP-2005-233321-A